Input buffer circuit

ABSTRACT

When an input terminal, which is connected to an input transistor, is opened, a transistor having an emitter connected to the input transistor conducts to supply constant current to a constant current source which is connected to the emitter of the input transistor. Therefore, load current of a reference circuit connected to the constant current source is not changed even if the input terminal is opened. As the result, current values of all constant current sources receiving voltage from the reference circuit are not changed so that the internal circuit of a semiconductor integrated circuit device can be stably operated.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improvement in an input stage of a semiconductor integrated circuit apparatus employing emitter-coupled logic (hereinafter referred to as ECL).

2. Description of the Background Art

FIG. 1 is a circuit diagram showing a conventional ECL circuit. A buffer circuit 1 is formed by a transistor Q₁ and a constant current source circuit 2. The transistor Q₁ is an emitter follower buffer NPN transistor. The transistor Q₁ has a base connected to an input terminal 3, a collector connected to a first power supply line 4 and an emitter connected to the constant current source circuit 2. The transistor Q₁ derives a signal which is responsive to an input signal received in the input terminal 3 at its emitter. The constant current source circuit 2 is a constant current load for supplying constant current to the transistor Q₁. The constant current source circuit 2 is formed by a transistor Q₂ and a resistor R₁. The transistor Q₂ is an NPN transistor, which has a base connected to a reference voltage input terminal 5, a collector connected to the emitter of the transistor Q₁ and an emitter connected to a second power supply line 6 through the resistor R₁.

An ECL differential pair 7 is formed by transistors Q₃ and Q₄. The transistors Q₃ and Q₄ are prepared by NPN transistors, emitters of which are commonly connected with each other. The transistor Q₃ has a base connected to the emitter of the transistor Q₁ and a collector connected to the first power supply line 4 through the resistor R₂ as well as to an output terminal 8. The transistor Q₄ has a base connected to a logic comparison reference voltage input terminal 9 and a collector connected to the first power supply line 4. The logic comparison reference voltage input terminal 9 is set at a potential value evaluated by adding the base-to-emitter potential of the transistor Q₁ to an intermediate potential between a high-level signal and a low-level signal supplied to the input terminal 3. The transistor Q₃ is made conductive/nonconductive depending on whether or not a signal level supplied to its base is higher than the potential of the logic comparison reference voltage input terminal 9, so that a high-level or low-level signal is outputted from the output terminal 8.

A constant current source circuit 10 is adapted to supply constant current to the ECL differential pair 7, and is formed by a transistor Q₅ and a resistor R₃. The transistor Q₅ is prepared by an NPN transistor, which has a base connected to the base of the transistor Q₂, a collector connected to the emitter common junction of the transistors Q₃ and Q₄ and an emitter connected to the second power supply line 6 through a resistor R₇.

Operation of the circuit in the aforementioned structure will now be described. When the input terminal 3 receives a high-level signal, the transistor Q₃ of the ECL differential pair 7 is supplied in its base with a high-level signal which is higher than the potential at the logic comparison reference voltage input terminal 9 through the buffer transistor Q₁, whereby the transistor Q₃ enters a conducting state and the transistor Q₄ enters a nonconducting state. Current flows to the resistor R₂ to cause voltage drop, and hence a low-level signal is outputted from the output terminal 8.

When the input terminal 3 receives a low-level signal, on the other hand, the transistor Q₃ of the ECL differential pair 7 is supplied in its base with a low-level signal which is lower than the potential at the logic comparison reference voltage input terminal 9 through the buffer transistor Q₁, whereby the transistor Q₃ enters a nonconducting state and the transistor Q₄ enters a conducting state. Since no current flows to the resistor R₂ to cause no voltage drop in this case, a high-level signal is outputted from the output terminal 8.

As hereinabove described, the transistor Q₁ is regularly in a conducting state when a high-level or low-level signal is received in the input terminal 3, to level-shift the potential of the high-level or low-level signal by its base-to-emitter voltage to derive the same at the emitter. In such a normal operation state, current to the constant current source circuit 2 is supplied from the first power supply line 4. Assuming that the constant current supplied to the constant current source circuit 2 is represented by I₀ and the grounded emitter current amplification factor of the transistor Q₂ is 100, current of I₀ /100 is supplied to the base of the transistor Q₂ from a reference circuit (not shown).

FIG. 2 is a circuit diagram showing another conventional ECL circuit, wherein the constant current source circuit 2 shown in FIG. 1 is formed by a current mirror circuit. The constant current source circuit 2 comprises a transistor Q₂, a resistor R₃ and a diode D₁. The diode D₁ is equivalent to a transistor whose base and collector are interconnected with each other. The transistor Q₂ has a base connected to a reference voltage input terminal 5 through the resistor R₃ as well as to a second power supply line 6 through the diode D₁ and an emitter connected to the second power supply line 6. This structure is so designed that a reference circuit (not shown), which is connected to the reference voltage input terminal 5, has reference voltage of V_(REF) and supplies reference current of I₀₁ {I₀₁ =(V_(REF) -V_(F)(D1) /R₃ }. V_(F)(D1) shows a voltage drop of diode D₁.

In the conventional semiconductor integrated circuit device having the aforementioned structure, the transistor Q₁ enters a nonconducting state when the input terminal 3 is opened, whereby the constant current I₀ to the constant current source circuit 2 cannot be supplied from the first power supply line 4. Thus, no collector current flows in the transistor Q₂. Therefore, the transistor Q₂ operates as a diode formed by the base and the emitter. Thus, load current of the reference circuit, which is base current of the transistor Q₂ in the circuit shown in FIG. 1, is increased from I₀ /100 to I₀, whereby output voltage of the reference circuit is changed. In the circuit shown in FIG. 2, on the other hand, the current I₀₁ flowing in the diode D₁ is divided by the diode D₁ and the transistor Q₂, to cause change in the anode potential of the diode D₁, which is reference voltage of the constant current circuit 10. Consequently, all of the constant current sources receiving these reference voltages are changed in current value, to cause deviation in potential of an internal circuit when the semiconductor integrated circuit device is used under the condition that the input terminal 3 is opened.

SUMMARY OF THE INVENTION

The present invention provides an input buffer circuit for buffering an input signal to a desired circuit, wherein the input buffer circuit and the desired circuit are formed as a semiconductor integrated circuit device. The inventive input buffer circuit comprises first and second power sources; a buffer transistor having a base which is supplied with the input signal, a first electrode connected to the first power source and a second electrode from which a signal responsive to the input signal is derived to be supplied to the desired circuit; a first constant current source connected between the second electrode of the buffer transistor and the second power source as a current load; a third power source connected to the first constant current source as a reference for defining a current flowing in the first constant current source; and a clamp circuit for clamping a potential of the second electrode of the buffer transistor or a control electrode or the base of the buffer transistor at a predetermined potential level.

Accordingly, it is an object of the present invention to provide an input buffer circuit employed for a semiconductor integrated circuit device, which causes no change in value of current flowing to a constant current source even if the input buffer circuit is used under condition that its input terminal is opened, so that an internal circuit is subjected to no potential deviation to perform stable operation.

These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 and FIG. 2 are circuit diagrams showing conventional input buffer circuits;

FIG. 3 is a circuit diagram showing an embodiment of the present invention; and

FIG. 4 to FIG. 12 are circuit diagrams showing other embodiments of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 is a circuit diagram showing an embodiment of an input buffer circuit according to the present invention. The circuit is formed as a semiconductor integrated circuit device. This embodiment is different from the conventional ECL circuit of FIG. 1 in such a point that a clamp circuit 11 is connected to an emitter of a transistor Q₁, for clamping the emitter potential of the transistor Q₁ at a predetermined potential level. The clamp circuit 11 is formed by a transistor Q₆ and a constant voltage source 12. The transistor Q₆ is a clamp NPN transistor, which has a base connected to the constant voltage source 12, a collector connected to a first power supply line 4 and an emitter connected to the emitter of the transistor Q₁ respectively. An output potential of the clamp circuit 11, i.e., the emitter potential of the transistor Q₆ is set at a level lower than a low-level signal which may be received in an input terminal 3, to exert no influence on output.

Operation of the circuit is the aforementioned structure will now be described. When a high-level or low-level signal is inputted in the input terminal 3, only the transistor Q₁ conducts while the transistor Q₆ enters a nonconducting state since the transistors Q₁ and Q₆ form an ECL differential pair. Thus, the operation in this case is similar to that of the conventional ECL circuit shown in FIG. 1.

When the input terminal 3 is opened, the transistor Q₁ is made nonconductive while the transistor Q₆ is made conductive, whereby the emitter potential of the transistor Q₁ is clamped at the output potential of the clamp circuit 11. Therefore, constant current I₀ to a constant current source circuit 2 is supplied through the transistor Q₆. Thus, assuming that the grounded emitter current amplification factor of a transistor Q₂ is 100, current of I₀ /100 is supplied to the base of the transistor Q₂ from a reference circuit (not shown) which is connected to a reference voltage input terminal 5, similarly to the case of inputting a high-level or low-level signal in the input terminal 3, even if the input terminal 3 is opened. Thus, base current of the transistor Q₂, which is load current of the reference circuit, is not changed even if the semiconductor integrated circuit device is used under the condition that the input terminal 3 is opened. Therefore, reference voltage inputted in the reference voltage input terminal 5 is not changed but remains constant, whereby current values of all constant current source circuits, including a constant current source circuit 10, to which the reference voltage is applied, remain constant. As the result, the semiconductor integrated circuit device can be stably operated with no regard to the state of connection of the input terminal 3.

Also when the constant current source circuit 2 shown in FIG. 3 is implemented by a current mirror circuit (FIG. 12) which is similar to that shown in FIG. 2, the emitter voltage of the transistor Q₁ is clamped at the output voltage of the clamp circuit 11 if the input terminal 3 is opened. Thus, the base potential of the transistor Q₂ is not changed since the current I₀ flows in the transistor Q₂ even if the input terminal 3 is opened. As the result, the current value of the constant current source 10, receiving the anode potential of the diode D₁ as reference voltage, is not changed and remains constant, whereby the semiconductor integrated circuit device can be stably operated with no regard to the state of connection of the input terminal 3.

FIG. 4 is a circuit diagram showing exemplary structure of the constant voltage source 12 in the above embodiment. Referring to FIG. 4, the constant voltage source 12 is formed by a constant current source circuit consisting of a resistor R₄ and a transistor Q₇, and a resistor R₅. The transistor Q₇ has a base connected to a reference voltage input terminal 5, an emitter connected to a second power supply line 6 through the resistor R₄ and a collector connected to the base of the transistor Q₆ as well as to a first power supply line 4 through the resistor R₅. In this case, current flowing in the resistor R₅ can be made constant by the constant current source circuit formed by the reference voltage input terminal 5, the transistor Q₇ and the resistor R₄. Thus, a voltage drop at the resistor R₅ can be made constant and the base voltage of the transistor Q₆ can be maintained constant, to attain an effect similar to that of the above embodiment. Further, the resistor R₅ may be replaced by a diode series circuit or a series circuit of a diode and a resistor.

FIG. 5 is a circuit diagram showing still another embodiment of the present invention. While the clamp circuit 11 is formed by the transistor Q₆ and the constant voltage source 12 in each of the embodiments shown in FIGS. 3 and 4, a clamp circuit 11 is formed by a diode series circuit in the embodiment shown in FIG. 5. In this diode series circuit, an anode is connected to a first power supply line and a cathode is connected to an emitter of a transistor Q₁. When an input terminal 3 is opened, the diode series circuit is turned on so that emitter voltage of the transistor Q₁ can be clamped at a predetermined potential level by a voltage drop of the diodes, to thereby attain an effect similar to those of the aforementioned embodiments. The diode series circuit may be replaced by a series circuit of a diode and a resistor.

Although the clamp circuit 11 is connected to the emitter of the transistor Q₁ so that the constant current I₀ flows in the constant current circuit 2 with no regard to the input state of the input terminal 3 in each of the aforementioned embodiments, a similar effect can also be attained by connecting a clamp circuit 11 to the base of a transistor Q₁, as shown in FIG. 6. In this case, the potential of a constant voltage source 12 is set at a level lower than a low level of an input signal, to exert no influence on output when a high-level or low-level signal is received in the input terminal 3. When the input terminal 3 is opened, the base potential of the transistor Q₁ is clamped at the output potential of the clamp circuit 11. Therefore, the transistor Q₁ maintains its conducting state and constant current I₀ to a constant current source circuit 2 is supplied from a first power supply line 4, to attain an effect similar to those of the aforementioned embodiments.

The constant voltage source 12 in FIG. 6 may be formed by resistors R₄ and R₅ and a transistor Q₇, as shown in FIG. 7, in a similar manner to that shown in FIG. 4. Further, the clamp circuit 11 in FIG. 6 may be formed by a diode series circuit, as shown in FIG. 8, in a similar manner to that shown in FIG. 5.

Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims. 

What is claimed is:
 1. A semiconductor integrated circuit device, comprising:a buffer transistor, said buffer transistor's base being connected to an input terminal; clamp circuit means, having an output connected to the emitter of said buffer transistor, for clamping the emitter potential of said buffer transistor at a predetermined potential level, said output of said clamp circuit means having a voltage which is maintained at a level lower than that of a low-level signal which may be received by the input terminal; a power supply line connected to an input of said clamp circuit means, said power supply line being connected to the collector of said buffer transistor; and a first constant current source connected to the emitter of said buffer transistor.
 2. A semiconductor integrated circuit device according to claim 1, wherein:said clamp circuit means comprises a constant voltage source connected to the base of a transistor, the emitter of said transistor connected to the emitter of said buffer transistor.
 3. A device according to claim 2, whereinsaid constant voltage source comprises a plurality of diodes in series.
 4. A device according to claim 2, wherein:said constant voltage source comprises a diode and a resistor in series.
 5. A semiconductor integrated circuit device according to claim 1, wherein:said clamp circuit means comprises a second transistor, the emitter of said second transistor connected to the emitter of said buffer transistor; and a third transistor, the collector of said third transistor connected to the base of said second transistor.
 6. A device according to claim 5, whereinsaid clamp circuit means further comprises a resistor connected between said first power source line and the base of said second transistor.
 7. A device according to claim 5, whereinsaid clamp circuit means further comprises a series circuit of a diode and a resistor connected between said first power source line and the base of said second transistor.
 8. A device according to claim 5, whereinsaid clamp circuit means further comprises a diode series circuit connected between said first power source line and the base of said second transistor.
 9. A semiconductor integrated circuit device according to claim 1, wherein:said clamp circuit means comprises a plurality of diodes connected in series, the output of said plurality of diodes connected to the emitter of said buffer transistor.
 10. A device according to claim 1, wherein said first constant current source comprises:a resistor R₁, and a transistor Q₂ having a base connected to a voltage input terminal, the collector of transistor Q₂ connected to the emitter of said buffer transistor and the emitter of transistor Q₂ connected to resistor R₁ and to a second power supply line.
 11. A device according to claim 10, further comprising:a second constant current source connected to the first constant current source.
 12. A semiconductor integrated circuit device according to claim 11, wherein the second constant current source comprises:a resistor R₇ and a transistor Q₅ having a base connected to the base of the transistor Q₂, and the emitter of transistor Q₅ connected to the second power supply line through said resistor R₇.
 13. A device according to claim 12, further comprising:an emitter coupled logic differential pair connected to the collector of said transistor Q₅.
 14. A device according to claim 13 wherein:the emitter logic coupled differential pair comprise transistors Q₃ and Q₄, the collector of the transistor Q₃ connected to an output terminal and connected to the first power supply line through a resistor R₂, and the base of the transistor Q₄ connected to a logic comparison input voltage terminal and the base of transistor Q₃ connected to the emitter of said buffer transistor.
 15. A device according to claim 1, wherein said first constant current source comprises:a diode D₁, a resistor R₃ and a transistor Q₂, the collector of the transistor Q₂ connected to the emitter of said buffer transistor, the emitter of the transistor Q₂ connected to a second power supply line and the base connected to a reference voltage input terminal through said resistor R₃, the base of transistor Q₂ connected to the second power supply line through said diode D₁. 